The heterojunction bipolar transistor (HBT) is an improvement of the bipolar junction transistor (BJT). HBT devices can handle signals of very high frequencies such as up to several hundred gigahertz, for example. Hence, HBT technology has certain advantages in modern fast circuits, such as radio frequency (RF) systems. HBTs made of Indium Phosphide and/or Indium Gallium Arsenide (GaAs) have advantages for use in optoelectronic integrated circuits. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters.